I think you described it excellently when you said “20+ years of speech experience and muscle memory.” We have such a strong reflex to talk in the way that’s natural, and I think social expectations and social anxiety fit in. Dealing with the anxiety/embarrassment of stuttering is one thing, but deliberately changing the way I spoke in the moment when i was in a conversation with someone... idk it was really hard. It was honestly just so hard that I got lazy and gave up. I had to psych myself up every time and even then I could only manage just a few very minimal voluntary stutters. But I know people who have no problem with it, so maybe it’s just me. I was a speech path major so one time I was in a class where the professor asked the (mostly) non stuttering students to do a pseudo stuttering exercise and most people found it hard but my one friend had no problem with it. She was like “yeah I just ordered food at a restaurant and stuttered, what’s the big deal?” So I think it’s a personality thing. P.s. might have to do with the fact that my stuttering is relatively mild most of time time and I do a lot of covert stuttering so I don’t often experience people’s reactions. Still have a butt load of anxiety about it but all in all it’s minor thing.